194 INVERTEBRATE ZOOLOGY 



SARCODINA 



A NAKED RHIZOPOD. AMOEBA 



The amoeba is a jelly-like, single-celled animal which may be 

 found in stagnant water attached to submerged objects, or in 

 bottom sediment; it is also often found in moist, damp places 

 which are not under water. The animals are very variable in 

 size, the largest being within the range of the unaided vision, 

 the smallest species requiring high powers of the microscope 

 to detect. 



Mount on a slide a drop of water with sediment or scrapings 

 from a submerged leaf or stick containing amoebas, and find 

 one. Observe its shape and granular appearance. From time 

 to time the shape of the body changes by the thrusting out 

 of projections called pseudopodia. Observe the formation of 

 pseudopodia. 



Exercise l. Draw several outlines of the animal, showing its 

 shape at different times. 



Observe the structure of the body. The protoplasm forming 

 it will be seen to be divisible into two layers, the ectosarc 

 and the entosarc; the former is the clear, transparent layer 

 which forms the periphery of the body ; the latter is the gran- 

 ular, translucent mass which forms the remainder of it. The 

 ectosarc is of firmer consistency than the entosarc and secretes 

 a delicate cuticula on its outer surface. When a pseudo- 

 podium begins to form, it consists at first of ectosarc alone, 

 but entosarc finally enters it as it grows larger. The entire 

 body will often flow into a single pseudopodium, in which case 



